The exo-methylenepenam compound of the present invention is an important intermediate for synthesizing, for example, a xcex2-lactamase inhibitor (Bawldwin et al, J. Chem. Soc., Chem. Commun., 1987, 81, S. Torii et al., Antibit. Chem. Lett., 1993, 3, 2253).
A process is already known for preparing the exo-methylenepenam compound of the invention which is represented by the general formula (2), by the decarboxylation Pummerer-type rearrangement reaction of penam-2-carboxylic acid derived from penicillin as illustrated in Diagram (A) (Bawldwin et al., J. Chem. Soc., Chem. Commun., 1987, 81), whereas this process comprises as many as eight reaction steps, and is as low as up to 6% in overall yield and by no means feasible. 
(wherein Rxe2x80x2xe2x95x90PhOCH2CONH, Rxe2x80x3xe2x95x90CH2C6H4NO2-p)
Also known are a synthesis process wherein an allenyl xcex2-lactam compound obtained from penicillin is subjected to acid hydrolysis, followed by intramolecular cyclization (S. Torii et al., Tetrahedron Lett., 1991, 32, 7445) as shown in Diagram (B), and a synthesis process wherein an allenyl xcex2-lactam compound is subjected to a reductive cyclization reaction (S. Torii et al., Synlett., 1992, 878, S. Torii et al., Chemistry Express, 1992, 7, 885, J. Chem. Soc., Chem. Commun., 1992, 1793). These processes nevertheless have various problems such as cumbersomeness of the reaction procedure for industrial operation since the reaction is conducted via an unstable allene compound as an intermediate. 
(wherein R1xcx9cR3 are same as above, R4 is aryl group which may have a substituent)
Further known is a synthesis process wherein a halogenated xcex2-lactam compound derived from penicillin is subjected to reduction for cyclization (Chemistry Letters, 1995, 709, JP-8-245,629 A). However, this process comprises many reaction steps compared with other processes because of use of the halogenated xcex2-lactam compound as an intermediate. Thus, reactions which are more practical are desired. 
(wherein R1xcx9cR4 are same as above)
An object of the invention is to provide a process adapted to produce an exo-methylenepenam compound of the formula (2) from the cephem compound of the formula (1) in a high yield with a high purity through a safe and simplified procedure by developing a novel metal reduction system.
The present invention further provides a process for preparing an exo-methylenepenam compound represented by the formula (2) wherein cephem compound represented by the formula (1) is reduced with a metal having a standard oxidation-reduction potential of up to xe2x88x920.3 (V/SCE) in an amount of at least one mole per mole of the cephem compound and with a metal compound having a higher standard oxidation reduction potential than the metal in an amount of 0.0001 to 10 moles per mole of the cephem compound to obtain the exo-methylenepenam compound 
wherein R1 is a hydrogen atom, amino or protected amino, R2 is a hydrogen atom, halogen atom, lower alkoxyl, lower acyl, lower alkyl, hydroxyl, protected hydroxyl or lower alkyl having hydroxyl or protected hydroxyl as a substituent, R3 is a hydrogen atom or carboxylic acid protective group, X is a halogen atom, lower alkylsulfonyloxy, substituted lower alkylsulfonyloxy, arylsulfonyloxy, substituted arylsulfonyloxy, halogenated sulfonyloxy or substituted halogenated sulfonyloxy 
wherein R1, R2 and R3 are as defined above.
Examples of groups mentioned herein are as follows.
Exemplary of the protected amino represented by R1 are amido groups such as phenoxyacetamido, p-methylphenoxyacetamido, p-methoxyphenoxyacetamido, p-chlorophenoxyacetamido, p-bromophenoxyacetamido, phenylacetamido, p-methylphenylacetamido, p-methoxyphenylacetamido, p-chlorophenylacetamido, p-bromophenylacetamido, phenylmonochloroacetamido, phenyldichloroacetamido, phenylhydroxyacetamido, thienylacetamido, phenylacetoxyacetamido, xcex1-oxophenylacetamido, benzamido, p-methylbenzamido, p-methoxybenzamido, p-chlorobenzamido, p-bromobenzamido, phenylglycylamido, phenylglycylamido having protected amino, p-hydroxyphenylglycylamido, p-hydroxyphenylglycylamido having protected amino and/or protected hydroxyl, etc.; imido groups such as phthalimido, nitrophthalimido, etc., in addition to the groups disclosed in Theodora W. Greene, 1981, xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d (hereinafter referred to merely as the xe2x80x9cliteraturexe2x80x9d), Chap. 7 (pp. 218xcx9c287). Examples of protective groups for the amino of phenylglycylamido group and p-hydroxyphenylglycylamido group are those disclosed in the literature, Chap. 7 (pp. 218xcx9c287). Examples of protective groups for the hydroxyl of p-hydroxyphenylglycylamido group are those disclosed in the literature, Chap.2 (pp. 10xcx9c72).
Exemplary of the lower alkoxyl represented by R2 are straight-chain or branched C1xcx9c4 alkoxyl groups such as methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, isobutoxy, sec-butoxy and tert-butoxy groups.
Exemplary of the lower acyl represented by R2 are straight-chain or branched C1xcx9c4 acyl groups such as formyl, acetyl, propionyl, butyryl and isobutyryl.
Examples of lower alkyl represented by R2 are straight-chain or branched C1xcx9c4 alkyl group such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl or tert-butyl. Examples of protective groups for the protected hydroxyl in the lower alkyl represented by R2 and substituted with hydroxyl or protected hydroxyl, and for the protected hydroxyl represented by R2 are those disclosed in the literature, Chap. 2 (pp. 10xcx9c72). The substituted lower alkyl represented by R2 may have as its substituent(s) one or at least two same or different groups selected from among hydroxyl and the protected hydroxyl groups. Such substituent(s) may be positioned on at least one carbon atom of the alkyl.
Exemplary of the carboxylic acid protecting group represented by R3 are allyl, benzyl, p-methoxybenzyl, p-nitrobenzyl, diphenylmethyl, trichloromethyl, tert-butyl, and those disclosed in the literature, Chap. 5 (pp. 152xcx9c192).
Examples of halogen atom represented by R2 and X are fluorine, chlorine, bromine or iodine atom.
Examples of lower alkylsulfonyloxy or substituted lower alkylsulfonyloxy are methanesulfonyloxy, trifluoromethanesulfonyloxy and trichloromethanesulfonyloxy. Examples of arylsulfonyloxy or substituted arylsulfonyloxy are benzenesulfonyloxy and toluenesulfonyloxy. Examples of halogenated sulfonyloxy are fluorosulfonyloxy.
The cephem compound represented by the formula (1) for use as a starting material of the present invention can be prepared for example by the following method.
More specifically, as disclosed in JP 49-116,095 A, the compound of the formula (1) is prepared by reacting 3-hydroxycephem compound of the formula (3) with a reactive chlorine compound (phosphorus trichloride, phosphorus oxychloride, etc.) in dimethylformamide. 
(wherein R1, R2 and R3 are as defined in above.)
The cephem compound of the formula (1) thus obtained can be converted to an exo-methylenepenam compound of the formula (2) by reacting the compound (1) with a metal having a standard oxidation-reduction potential of up to xe2x88x920.3 (V/SCE) in an amount of at least one mole per mole of the compound (1) and a metal compound having a higher standard oxidation-reduction potential than the metal in an amount of 0.0001 to 10 moles per mole of the compound (1). V/SCE shows oxidation-reduction potential based on a standard caramel electrode.
More specifically, the reaction is conducted in a suitable solvent. Examples of solvents useful in the reaction are lower alkyl esters of lower carboxylic acids such as methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate and ethyl propionate, ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, methyl butyl ketone, methyl isobutyl ketone and diethyl ketone, ethers such as diethyl ether, ethyl propyl ether, ethyl butyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, methyl cellosolve and dimethoxyethane, cyclic ethers such as tetrahydrofuran, dioxane and dioxolan, nitriles such as acetonitrile, propionitrile, butyronitrile, isobutyronitrile and valeronitrile, substituted or unsubstituted aromatic hydrocarbons such as benzene, toluene, xylene, chlorobenzene and anisole, hydrocarbon halides such as dichloromethane, chloroform, dichloroethane, trichloroethane, dibromoethane, propylene dichloride, carbon tetrachloride, aliphatic hydrocarbons such as pentane, hexane, heptane and octane, cycloalkanes such as cyclopentane, cyclohexane, cycloheptane and cyclooctane, amides such as dimethylformamide and dimethylacetamide, cyclic amides such as N-methylpyrrolidinone, dimethylsulfoxide, etc. These solvents are used singly or in admixture of at least two of them. These solvents may contain water as required. These solvents are used in an amount of about 10 to about 200 liters, preferably about 20 to about 100 liters, per kilogram of the compound of the formula (1). The reaction is conducted usually at xe2x88x9278xc2x0 C. to 60xc2x0 C., preferably xe2x88x9240xc2x0 C. to 30xc2x0 C. The reaction of the invention proceeds satisfactorily even around room temperature. Further, when required, the reaction can be conducted within a closed container or in an inert gas such as nitrogen gas.
Examples of metals having a standard oxidation-reduction potential of up to xe2x88x920.3 (V/SCE) are magnesium, aluminum, zinc, iron, nickel, tin, lead, etc., among which magnesium, aluminum, zinc and tin are desirable to use. The shape of these metals is not limited particularly but can be any of a wide variety of forms such as powder, plate, foil, lump and wire. Preferably, the metal to be used is in the form of a powder or foil. The particle size of the powdery metal is preferably about 100 to about 300 mesh although variable over a wide range. These metals are used usually in an amount of about 1 to about 50 moles, preferably about 1 to about 10 moles, per mole of the compound of the formula (1).
Examples of metal compounds having a higher standard oxidation-reduction potential than the above metals are lead compounds (such as lead fluoride, lead chloride, lead bromide, lead iodide and like lead halides, lead nitrate, lead sulfate, lead perchlorate, lead borate, lead carbonate, lead phosphate and like inorganic salts of lead, lead acetate, lead oxalate, lead stearate and like fatty acid salts of lead, lead oxide and lead hydroxide), copper compounds (such as copper fluoride, copper chloride, copper bromide, copper iodide and like copper halides, copper nitrate, copper sulfate, copper perchlorate, copper borate, copper carbonate, copper phosphate and like inorganic salts of copper, and copper oxalate), titanium compounds (such as titanium fluoride, titanium chloride, titanium bromide, titanium iodide and like titanium halides, and titanium nitrate, titanium sulfate and like inorganic salts of titanium), bismuth compounds (such as bismuth fluoride, bismuth chloride, bismuth bromide, bismuth iodide and like bismuth halide, bismuth nitrate, bismuth sulfate and like inorganic salts of bismuth), antimony compounds (such as antimony fluoride, antimony chloride, antimony bromide, antimony iodide and like antimony halides, antimony sulfate and like inorganic salts of antimony, and antimony oxide), and nickel compounds (such as nickel fluoride, nickel chloride, nickel bromide, nickel iodide and like nickel halides, nickel nitrate, nickel sulfate, nickel perchlorate, nickel borate, nickel carbonate, nickel phosphate and like inorganic salts of nickel, nickel acetate and like fatty acid salts of nickel. These metal compounds may be used singly or as a mixture of at least two of them. These metal compounds are used usually in an amount of 0.0001 to 30 moles, preferably 0.001 to 10 moles, per mole of the compound of the general formula (1).
Examples of combinations of metals up to xe2x88x920.3 (V/SCE) in standard oxidation-reduction potential and metal compounds having a higher standard oxidation-reduction potential are Al/Pb compound, Al/Bi compound, Zn/Pb compound, Zn/Bi compound, Mg/Bi compound, Mg/Cu compound, Sn/Ti compound, Sn/Bi compound, Sn/Sb compound, etc., among which the combinations of Al/Pb compound, Al/Bi compound and Zn/Bi compound are preferred.
It is possible to add an acid to the reaction system in order to proceed the reaction smoothly. Examples of acids are mineral acid such as hydrochloric acid and sulfuric acid, Louis acid such as aluminum chloride.
The exo-methylenepenam derivative of the formula (2) obtained can be isolated by a usual purification procedure.